Recently, active research is being done on the design of an antenna using a metamaterial. The metamaterial indicates material which has a specific unit structure periodically arranged and an electromagnetic property not existing in the natural world.
From among several kinds of metamaterials, a metamaterial having a randomly controllable dielectric constant magnetic permeability has been in the spotlight. Representatively, a material called ‘Negative Refractive Index (NRI)’ or ‘Left-Handed Material (LHM)’ has both the valid dielectric constant and the magnetic permeability of a negative value and complies with the left hand rule in the electric field, the magnetic field, and the electric wave traveling direction. If the metamaterial is applied to an antenna, the performance of the antenna is improved by the characteristics of the metamaterial.
A metamaterial structure applied to an antenna representatively includes a Composite Right/Left Handed Transmission Line (CRLH-TL) structure. A 0-th order resonant mode (i.e., one of the characteristics of the structure) is a resonant mode in which the propagation constant becomes 0. In the 0-th order resonant mode, the wavelength becomes infinite, and no phase delay according to the transmission of electric waves is generated. The resonant frequency of this mode is determined by the parameters of the CRLH-TL structure and thus very advantageous in a reduction in the size of an antenna because it does not depend on the length of the antenna.
Of course, an antenna can be made using a first order resonant mode. In this case, the antenna can be designed to have a very low resonant frequency, while having the same radiation pattern as a common patch antenna.
Recently, there is a growing interest in a magneto-dielectric material capable of increasing the magnetic permeability. As a conventional method of decreasing the size of an antenna, there is a method using a substrate of a high dielectric constant. However, the method is disadvantageous in that the efficiency of an antenna is reduced and the bandwidth is narrowed because energy is confined in the substrate of a high dielectric constant. Meanwhile, if a substrate having a high magnetic permeability is used, the above problems can be solved and also the antenna can be reduced in size.
In order to fabricate the magneto-dielectric material, a metal structure responding to an external magnetic field is inserted into a common substrate. A Split Ring Resonator (SRR) is chiefly used as the structure. Current is induced into the SRR by an external magnetic field, and a magnetic field is generated by the induced current. Accordingly, the magnetic permeability is changed in response to the external magnetic field. The magnetic permeability has a resonating characteristic. The magnetic permeability is 1 or higher in a band under a resonant frequency, a negative value between the resonant frequency and a plasma frequency, and a positive value 1 or fewer over the plasma frequency. The band used as the magneto-dielectric material is a region under the resonant frequency.